Reverse magnetron having an output circuit employing mode absorbers in the internal cavity



Nov. 18, 1969 A. w. cooK 3,479,556

REVERSE MAGNETRON HAVING AN OUTPUT CIRCUIT EMPLOYING MODE ABSORBERS IN THE-INTERNAL CAVITY Filed Sept. 27, 196'? I. It L N l 0 I FIG.2

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AT RN EY United States Patent O US. Cl. 31539.51 5 Claims ABSTRACT OF THE DISCLOSURE A reverse magnetron tube is disclosed. The tube includes a circular array of anode vane resonators surrounded by a cathode emitting structure to define annular magnetron interaction region in the space between the vanes and the surrounding cathode emitter. The vane array is arranged around the outside of a circular electric mode cavity resonator. A circular array of axially directed elongated coupling slots communicate through the walls of the circular electric mode cavity with alternate ones of the vane resonators for locking the 1r-mode of vane resonator system to the dominant circular electric mode in the internal cavity resonator. A tuning structure is provided in the internal cavity for tuning the output frequency of the tube. The circular electric mode resonator includes a fixed end wall having an elongated radially directed coupling slot communicating therethrough for coupling output wave energy from the circular electric mode of the cavity to a rectangular waveguide and thence through a wave permeable vacuum window to a load. A quarter-wave impedance matching transformer is provided between the coupling slot and the rectangular waveguide for impedance matching the coupling slot to the waveguide. An annular groove is provided within the cavity, encircling the end wall of the cavity, and a lossy mode absorbing ring is disposed in the groove for damping or loading down possibleresonant modes of the cavity having radially directed current flowing across the end wall of the cavity. In addition, a centrally disposed conductive post is provided in the cavity for moving a certain interfering TM mode out of the tunable band of the cavity.

DESCRIPTION OF THE PRIOR ART Heretofore, reverse magnetrons have been constructed employing a cylindrical output waveguide structure coupled to the circular electric mode internal resonator of the tube by means of a coupling plate having an annular coupling hole in alignment with the output waveguide. Such an output circuit is described and claimed in US. Patent 3,289,035 issued Nov. 29, 1966 and assigned to the same assignee as the present invention. One of the problems associated with utilizing a cylindrical output waveguide is that most utilization devices, which are to be connected to the output of the magnetron, employ rectangular waveguide input connections. Therefore, a circular electric mode to rectangular waveguide mode transition is typically required for converting the circular electric mode output of the tube to the conventional dominant TE transmission mode in the rectangular waveguide. Such transitions are relatively bulky, expensive and typically have relatively narrow operating bandwidths as of 6%.

3,479,556 Patented Nov. 18, 1969 Therefore, it is desirable to provide an output for the reverse magnetron which transforms the circular electric mode energy within the internal cavity of the tube to the dominent TE rectangular waveguide transmission mode in a relatively short distance with relatively wide bandwidth. It is preferred that the transition section be relatively small such that it may be readily accommodated as an integral part of the tube.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved reverse magnetron tube apparatus.

One feature of the present invention is the provision, in a reverse magnetron tube, of an output circuit which couples circular electric mode energy from the internal cavity of the tube to a rectangular Waveguide output and which employs a cavity mode absorption structure within the cavity for loading down possible competing modes of oscillation within the cavity.

Another feature of the present invention is the same as the preceding feature wherein the output coupling circuit includes a radially directed slot communicating through an end wall of the circular electric mode cavity resonator and a quarter-wave impedance transformer for matching the impedance of the coupling slot to the im pedance of the rectangular waveguide.

Another feature of the present invention is the same as any one or more of the preceding features wherein the circular electric mode resonator includes a central conductive post, axially directed of the resonator for tuning a certain undesired interfering cavity mode out of the tuning band of the tube.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view, partly broken away, of a reverse magnetron incorporating features of the present invention,

FIG. 2 is a plot of frequency in gHz. vs. cavity length in inches, showing the tunable band of the tube and the possible interfering and competing modes within the cavity,

FIG. 3 is a sectional view of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows, and

.FIG. 4 is an enlarged sectional view of a portion of the structure of FIG. 1 taken along line 4-4 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a reverse magnetron tube 1 incorporating features of the present invention. The tube includes a circular array of radially directed vane resonators 2 surrounded by a thermionic cathode emitter structure 3 to define an annular magnetron interaction gap 4 in the space between the tips of the vanes 2 and the cathode 3, A cylindrical circular electric mode resonator chamber 5 is coaxially disposed of the array of vane resonators 2 and is surrounded by the vane resonators 2. An array of axially directed elongated coupling slots 6 communicate through the side walls of the resonator chamber 5 with alternate ones of the anode resonators formed by the space between adjacent vane members 2. The slots 6 serve to lock the 1r-mode of Oscillation of the anode resonator system to the dominant circular electric mode, namely, the TE mode within the cavity 5.

A pair of mutually opposed annular magnetic poles 7 and 8 are disposed on opposite exial ends of the magnetron interaction region 4 for producing an axially directed magnetic field B within the magnetron interaction gap 4. A magnet structure, not shown, is provided for energizing the magnet poles 7 and 8.

The cavity resonator includes a fixed circular end wall 11, at one end thereof, and an annular tuning plate 12, as of copper, defining the other end wall of the cavity 5. An annular groove 13 encircles the fixed end plate 11 to define a short section of coaxial transmission line. A lossy mode absorbing ring 14, as of carbon impregnated alumina, is disposed in the bottom of the groove 13 for absorbing energy from certain possible competing modes of oscillation within the cavity 5 which have currents which tend to flow across the groove 13. However, the mode absorbing ring is not substantially coupled to the desired dominant circular electric mode of the cavity 5 since it has no radially directed currents tending to flow across the groove 13.

The cavity tuner plate 12 is moved axially of the cavity by means of a hollow tuning shaft 17. The tuning shaft 17 is extrenally threaded near its outer end for mating with threads of an axially captured nut 18. The tuning shaft 17 is also captured against rotation such that turning the nut 18 causes the shaft 17 to move in the axial direction 4, changing the length of the cavity 5 and, thus, tuning its resonance frequency over the band from 9.6 gHz. to 8.6 gHz. A suitable vacuum tight bellows ararngement, not shown, is provided interconnecting the shaft 17 and the remaining portion of the vacuum envelope 21 of the tube 1. The envelope 21 is evacuated to a suitably low pressure as of torr.

Referring now to FIG. 2, there is shown a plot of frequency vs. cavity length in inches for a typical circular electric mode resonator 5 dimensioned for operation at X hand. There is an interfering TM mode, namely the TM 2, 0 mode which interferes with the dominant desired TE 1,1 mode within the cavity. It has been found that this undesired TM mode can be shifted out of the tunable band of the cavity 5, without appreciably disturbing the desired TE, 1, 1 mode and without moving the other possible modes within the tunable band of the cavity, by placing a conductive post 22 on the center line of the cavity 5. This conductive post 22 is disposed coaxially of the cavity and interconnects the fixed end wall 11 with the tuning plunger 12 defining a moveable end wall of the cavity 5. The post 22 serves to substantially de-tune the undesired TM 2, 0 mode and to move its operating frequency to approximately 11.0 gHz., thus avoiding interference with the desired TE 1,1 mode. The remaining TE and TM modes, other than the desired TE 1, 1 mode, are heavily damped by means of the mode absorber 14. The tube is tunable from 9.6 gHz. to 8.6 gHz. without interference from these other possible competing modes of oscillation within the cavity. The tuning plate 12 includes a central opening in alignment with the hollow interior of the tuning shaft 17 to accommodate passage of the rod. 22 therethrough such that the tuning plunger 12 can slide axially of the rod 22.

Output R.F. energy is extracted from the dominant TE 1, 1 mode of the cavity 5 by means of a radially directed elongated coupling slot 25 communicating through the fixed end wall 11 of the cavity 5, as shown more clearly in FIG. 3. The coupling slot 25 extends across the region of the most intense current I for the dominant circular electric mode such that this mode is heavily coupled via the slot 25 to a rectangular waveguide 26 and thence through a broadband microwave window 27 to a load, not shown.

A quarter-wave length impedance matching section of waveguide 28, more clearly shown in FIG. 4, is provided for matching the relatively low impedance of the slot 25 to the relatively high impedance of the output rectangular waveguide .26. More specifically, the impedance of the slot is typically between 10 and 20 ohms and the impedance of the rectangular waveguide 26 is approximately 200 ohms, and the impedance matching section 28 is dimensioned to be a quarter-wavelength long and to have a characteristic impedance Z /Z -Z where Z is the characteristic impedance of Waveguide 26 and Z is the slot impedance. A convenient way to obtain the quarter-Wave impedance matching section 28 is to employ ridged waveguide with the ridges removed to form the rectangular waveguide section 26.

The advantage of the output circuit for the magnetron tube 1 is that it permits the dominant TE transmission mode to be obtained in rectangular waveguide 26 from a transition section 28 which is relatively short and which provides broadband operation as of 14%.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a magnetron tube apparatus, means forming a circular array of anode resonators, means forming a cathode emitter concentrically surrounding said anode resonators for defining an annular magnetron interaction region therebetween, means for producing an axially directed magnetic field in the magnetron interaction region, means forming a circular electric mode resonator chamber concentrically disposed inside said circular array of anode resonators, means forming a circular array of axially directed elongated coupling slots communicating between said circular electric mode resonator and said anode resonators for locking the 1r-II10dB of oscillation of said anode resonators to the circular electric mode of said resonator chamber, said circular electric mode resonator chamber including an end wall at one axial end thereof, coupling means communicating through said end wall for coupling radio frequency wave energy from a circular electric mode of said resonator chamber to a load, the improvement wherein, said circular electric resonator chamber includes, means forming an annular groove in the wall of said chamber, said groove encircling said end wall of said chamber, a mode absorber structure disposed in said annular groove for absorbing energy from the cavity modes having radial current components in said end wall.

2. The apparatus of claim 1 wherein said coupling means includes, a coupling slot communicating through said cavity end wall, a rectangular waveguide aligned with said coupling slot, an impedance matching means within said waveguide for matching the impedance of said coupling slot to the impedance of said waveguide, and a wave permeable gas tight window structure sealed across said waveguide.

3. The apparatus of claim 2 wherein said coupling slot is elongated in the radial direction of said end wall of said resonator chamber.

4. The apparatus of claim 1 wherein end wall of said resonator chamber is fixed in position relative to the side walls of said resonator chamber, means forming an axially directed conductive post centrally disposed of said chamber and extending lengthwise of said chamber, said post being fixed to said fixed end 'wall of said chamber, and means forming an annular moveable tuning plunger defining a moveable end wall of said resonator chamber, said plunger being moveable of said central post for tuning said resonator chamber and the frequency of the radio frequency output of the tube.

5. The apparatus of claim 2 wherein said impedance 5 6 matching means is an impedance matching section of 3,395,314 7/1968 Decker 31539.77 X Waveguide one quarter-Wave length long. 3,412,284 11/ 1968 Glenfield 3l539.77

References Cited HERMAN KARL SAALBACH, Primary Examiner UNITED STATES PATENTS 5 SAXFIELD CHATMON, JR., Assistant Examiner 2,817,823 12/1957 Okress 315-3953 X 2,901,666 8/1959 Sixsmith 315 s9.51

3,231,781 1/1966 Liscio 315-3977 X 31539.53, 39.61, 39.77; 3319l; 333-83 

